RTF is a global modification that replaces all towns in the game.

You can upgrade it to make an explosive ballista or an iron ballista, like in NCF, but change the color scheme to match the dwarven ballista from Heroes 7. Alternatively, it would be better to make a ballista with a gun crew from the dark dwarves instead of a ballista, or a dark cannon with magic, although you could also make a fire or electric one.